This study is designed to determine a recommended phase II dose of 5-fluorouracil plus eniluracil when given concurrently to patients with locally advanced cancers of the upper GI tract (pancreas, distal bile ducts, ampulla, duodenum). It will describe the non-dose limiting toxicities of this therapy and determine if the pharmacokinetics of oral eniluracil/5-FU are altered by concomitant upper GI radiation therapy. The study will preliminarily describe the radiographic response rate, the surgical resectability rate, overall and disease free survivals. Twelve blood samples for assay of study drugs and uracil plasma concentrations are drawn over 12 hours on treatment days 8 and 31. Also on days 8 and 31, urine is collected for 12 hours.